Heretofore, the technology using an anion exchange resin (hereinafter referred to as IER) has been known as a method of recovering a fluorine-containing emulsifier used for an emulsion polymerization of a fluoropolymer.
JP-B-47-51233 describes a method wherein latex resulting from emulsion polymerization is coagulated and washed; an emulsifier is collected as an aqueous solution; the aqueous solution thus obtained is evaporated to dryness; and then the fluorine-containing emulsifier is recovered with an organic solvent. Furthermore, its specification describes a recovery method of the fluorine-containing emulsifier with the use of an anion exchange resin.
U.S. Pat. No. 4,282,162 describes a method wherein a dilute aqueous solution of an emulsifier is brought into contact with a weakly basic anion exchange resin in a pH range of from 0 to 7 to adsorb the emulsifier on the resin and wherein it is desorbed with aqueous ammonia.
WO99/62830 describes a method of adding a nonionic or cationic surfactant to coagulation waste water of a fluoropolymer to stabilize fine particles of polytetrafluoroethylene (hereinafter referred to as PTFE) in the coagulation waste water, thereby preventing clogging of a packed column of IER.
JP-A-55-120630, U.S. Pat. No. 4,369,266 and DE 2,908,001 describe a method wherein coagulation waste water of PTFE is concentrated by means of an ultrafiltration method, and at the same time, a part of ammonium perfluorooctanoate (hereinafter referred to as APFO) used in the production of PTFE is recovered, and then APFO is adsorbed and recovered with IER.
JP-A-55-104651, U.S. Pat. No. 4,282,162 and DE 2,903,981 describe a method of adsorbing APFO on IER and then desorbing and recovering perfluorooctanoic acid with the use of a mixture of an acid and an organic solvent.
WO99/62858 describes a method wherein lime water is preliminarily added to coagulation waste water of a tetrafluoroethylene/perfluoro(alkyl vinyl ether)copolymer (hereinafter referred to as PFA) to adjust the pH to a level of from 6 to 7.5; then a metal salt such as aluminum chloride or iron chloride is added to coagulate the non-coagulated PFA; then the coagulates are mechanically separated and removed; thereafter the pH of the resultant waste water is adjusted to a level of at most 7 with sulfuric acid; and APFO is adsorbed and recovered by means of strongly basic IER.
JP-A-2001-62313 describes a method of desorbing APFO adsorbed on IER with the use of a mixture of water, an alkali and an organic solvent.
JP-A-2002-59160 describes a method of desorbing a fluorine-containing emulsifier adsorbed on IER with the use of a mixture of water, an alkali (especially, sodium hydroxide) and an organic solvent (especially, methanol, ethanol or acetonitrile).
Furthermore, JP-A-2002-58966 describes a method of concentrating and recovering a fluorine-containing emulsifier by means of a reverse osmosis membrane.
In addition, The Chemical Society of Japan, 76th Spring Meetings and The Chemical Society of Japan, 80th Fall Meetings have reported the technology of recovering perfluorooctanoic acid and its ammonium salt by use of a layered double hydroxide of aluminum and zinc.
WO02/10104A1 and WO02/10105A1 describe a method for recovering a fluorine-containing emulsifier by adding a bivalent metal and a trivalent metal to an aqueous solution containing a fluorine-containing emulsifier, thereby forming a layered double hydroxide.
However, in the methods using IER and the method using the reverse osmosis membrane, it is necessary to remove a suspended solid and/or a substance convertible to a suspended solid (hereinafter collectively referred to as the SS component) containing a non-coagulated fluorine-containing polymer, prior to contact with the IER or the reverse osmosis membrane. The removal of the SS component considerably affects a recovery efficiency of the fluorine-containing emulsifier, and there remain many problems in actual operations, including a problem that no quite effective method of removing the SS component has been found.
Furthermore, in the recovering method using the layered double hydroxide reported in The Chemical Society of Japan, 76th Spring Meetings and The Chemical Society of Japan 80th Fall Meetings, and the recovering method described in WO02/10104A1 and WO02/10105A1, there was a problem that a recycle efficiency was not sufficiently increased after recovery of the fluorine-containing emulsifier, because the other chemical substances such as the bivalent metal and the trivalent metal had to be added for recovering the fluorine-containing emulsifier.
Heretofore, in a case wherein a fluorine-containing emulsifier was recovered by evaporating to dryness, there were problems that an extremely large amount of energy was required and that the fluorine-containing emulsifier was also lost together with water during evaporation of water.
The present invention has been accomplished under such circumstances and it is an object of the present invention to provide a method for simply and efficiently recovering a fluorine-containing emulsifier from a low-concentration aqueous solution such as coagulation waste water of a fluoropolymer.